MAX17058_13 datasheet(7/16 Pages) MAXIM

Part #	MAX17058

Description	1-Cell/2-Cell Li ModelGauge ICs

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Manufacturer	MAXIM [Maxim Integrated Products]

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MAX17058/MAX17059

1-Cell/2-Cell Li+ ModelGauge ICs

7

Maxim Integrated

Detailed Description

ModelGauge Theory of Operation

The MAX17058/MAX17059 ICs simulate the internal, non-

linear dynamics of a Li+ battery to determine its state of

charge (SOC). The sophisticated battery model consid-

ers impedance and the slow rate of chemical reactions in

the battery (Figure 2).

The ModelGauge algorithm performs best with a custom

model, obtained by characterizing the battery at multiple

discharge currents and temperatures to precisely model

it. Contact Maxim if you need a custom model. At power-

on reset (POR), the ICs have a preloaded ROM model

that performs well for some batteries.

Fuel-Gauge Performance

In coulomb counter-based fuel gauges, SOC drifts

because offset error in the current-sense ADC measure-

ment accumulates over time. Instantaneous error can be

very small, but never precisely zero. Error accumulates

over time in such systems (typically 0.5%–2% per day)

and requires periodic corrections. Some algorithms cor-

rect drift using occasional events, and until such an event

occurs the algorithm’s error is boundless:

• Reaching predefined SOC levels near full or empty

• Measuring the relaxed battery voltage after a long

period of inactivity

• Completing a full charge/discharge cycle

The ModelGauge algorithm requires no correction events

because it uses only voltage, which is stable over time. As

the SOC accuracy without full/empty/relax shows the

algorithm remains accurate despite the absence of any

of the above events; it neither drifts nor accumulates

error over time.

To correctly measure performance of a fuel gauge as

experienced by end-users, exercise the battery dynami-

cally; accuracy cannot be fully determined from only

simple cycles.

Battery Voltage and State-of-Charge

The open-circuit voltage (OCV) of a Li+ battery uniquely

determines its SOC; one SOC can have only one value of

OCV. In contrast, a given VCELL can occur at many dif-

ferent values of OCV because VCELL is a function of time,

OCV, load, temperature, age, and impedance, etc.; one

value of OCV can have many values of VCELL. Therefore,

one SOC can have many values of VCELL, so VCELL can-

not uniquely determine SOC.

Figure 3 shows that VCELL = 3.81V occurs at 2%, 50%,

and 72% SOC.

Even the use of sophisticated tables to consider both

voltage and load results in significant error due to the

load transients typically experienced in a system. During

charging or discharging, and for approximately 30min

after, VCELL and OCV differ substantially, and VCELL has

been affected by the preceding hours of battery activity.

ModelGauge uses voltage comprehensively by using

voltage measured over a long period of time.

Figure 2. Block Diagram

Figure 3. Instantaneous Voltage Does Not Translate Directly to

SOC

STATE

MACHINE

(SOC)

I2C

INTERFACE

IC

GROUND

TIME BASE

(32kHz)

ADC (VCELL)

VOLTAGE

REFERENCE

BIAS

GND

CELL

VDD

SCL

SDA

ALRT

QSTRT

MAX17058

MAX17059

TIME (Hr)

100%

80%

60%

40%

20%

0%

012345678

3.4V

3.6V

3.8V

4.0V

4.2V

3.2V

3.81V = 2%

3.81V = 72%

3.81V = 50%

3.81V

VCELL

SOC

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